Al-SUBSTITUTION EFFECTS ON PHYSICAL PROPERTIES OF THE COLOSSAL MAGNETORESISTANCE COMPOUNS La 0.67 Ca 0.33 MnO 3 I.G. Deac a, *, L. Giurgiu b , A. Darabont a , R.V. Tetean a , M. Miron a , E. Burzo a a Faculty of Physics, "Babes-Bolyai" University, 3400 Cluj-Napoca, Romania b Institute of Izotopic and Molecular Technology, 3400 Cluj-Napoca, Romania We present a detailed study of the polycrystalline perovskite manganites La 0.67 Ca 0.33 Al x Mn 1-x O 3 (x = 0, 0.1, 0.15, 0.5) at low temperatures and high magnetic fields, including electrical resistance, magnetization, ac susceptibility. The static magnetic susceptibility was also measured up to 1000 K. All the samples show colossal magnetoresistance behavior and the Curie temperatures decrease with Al doping. The data suggest the presence of correlated magnetic clusters near by the ferromagnetic transition. This appears to be a consequence of the structural and magnetic disorder created by the random distribution of Al atoms. 1. Introduction In the last few years there has been great interest in the relation between the structural, magnetic, and transport properties of La 1-x Ca x MnO 3 and related rare earth manganate perovskites, following the observation of large magnetoresistance in these materials. 1,4 The occurrence of ferromagnetism had been attributed to the double-exchange (DE) interaction between Mn 3+ and Mn 4+ ions, and the richness of the phase diagrams 4 were naturally considered as a manifestation of the strong couplings among the spin, charge and Jahn-Teller (JT) lattice distortions. This coupling has recently been shown to commonly result in * Corresponding author. Tel.: +4-026-440-5300; fax: +4-026-459-1906. E-mail address: ideac@phys.ubbcluj.ro.